Frequency indicating instrument



1941- G. M. WRIGHT ET AL 4, 23

FREQUENCY INDIGATING INSTRUMENT Fil'ed Dc. .15, 1958 INVENTORS GEQRGE MAURICE WRIGHT BY CYR/L AU IN I u z a I ATTORNEY Patentecl Aug. 26, 1941 George Maurice Cyril Austin,

- signers to Radio Corporation poration of Delaware Wright, woodham 'Ferrers, and Stockwell, London, England, as

of America, a cor- ApplicationDecember 13, issa seriai No. 245,454 1 In Great Britain January 1 1938 i i 3:01aims. (01. 250-49) ,Thisiinvention' relates .to wave meters and other frequency indicatinginstrumentsfadapted for use for. indicatingthe frequency of radio and other high frequency'currents, and haslfor its object to provide. improved frequencyindieating instruments rmgn effective discrimination and thereforeLcapable of being. read to a high degree of closeness. f g

The at present usual formof frequency indicating instrument employed for measuring the frequency of cur-rents in radio telegraph and telephone apparatus}.consists essentially of a carefully calibrated adjustable parallel tuned circuit (either the inductance or the capacity or both, of this circuit may be variable) and a voltage responsive indicator connected across it or, alternatively, a current responsive indicator connected in series with it. In use the calibrated circuit is adjusted to resonance, this operation being performed with the aid of the indications given by the voltage or current responsive indicator, as the case may be, and the frequency determined from a knowledge of the calibration of the tuned circuit.

This type of instrument possesses the defect that its effective discriminationi. e. its discrimination at the point at which it is read, namely resonanceis at a minimum, for at resonance the rate of change of indication given by the indicator per unit of change of the constants of the tuned circuit, is a minimum. In fact, at resonance, the tangent to the curve connecting adjustment of the tuned circuit with frequency is parallel to the frequency axis.

The principal object of the is to avoid this defect and the invention achieves this object by utilizing the fact that, at resonance, the rate of change of phase of the total current passing through a resonant oscillatory circuit relative to the phase of the applied E. M. F. is substantially at a maximum as the frequency is changed.

According to this invention a Wave meter or other frequency indicating instrument comprises an oscillatory circuit, 'an oscillograph capable of present invention deflection in two angularly related directions,

means responsive to the voltage effect produced by the rate of change of the total phase of current passing through said oscillatory circuit for producing deflection in one direction in said oscillograph and means responsive to the E. M. F.

of the frequency to be indicated for producing deflection in the other direction in said oscillograph.

Preferably the oscillograph is a cathode ray tube having a fluorescent screen and mutually perpendicular ray deflection means e. g. mutually perpendicular pairs of electrostatic deflecting plates. U f v .The inventionisil-lustrated in and furtherexplained infconnection with the accompanyin drawing, in which Fig.1 shows one embodiment of my invention,

Fig. 2 Yshows'a second embodiment, and

' Figs..3 and 4 are explanatoiy curves.

Referring to Fig. 1 which shows diagrammath cally one way of carrying out the invention; a frequency indicating instrument comprises a calibrated adjustable circuit comprising an inductance l and a condenser 2 in parallel. The circuit 1, 2 is connected by leads 3, 4 between one pair 5, 6 of electrostatic deflecting plates of a cathode ray tube of the fluorescent screen type and having two mutually perpendicular pairs 5, 6 and I, 8 of deflecting plates. The gun of the tube is represented by the dot 9. The input terminals H], II of the instrument are connected one (It) directly to one end of the tuned circuit and the other (H) through an impedance [2 to the other end. The impedance i2 is of a value comparable to the impedance of the parallel tuned circuit at resonance. The input terminals H), II are connected as shown to the other pair 1, 8 of deflecting plates.

With this arrangement, if the voltages fed to the two pairs of deflecting plates are equal in amplitude and the impedance is pure resistance, the pattern traced out by the cathode ray on the fluorescent screen will vary from a nearly circularly pattern, when the frequency applied at the input terminals is materially different from the resonant frequency of the tuned circuit, to a straight line making an angle of 45 to the direction of separation of the deflecting plates of either pair, when the input frequency is equal to the resonant frequency of the tuned circuit. For positions of even very slight mistune on either 'side of the resonance position this straight line becomes a'narrow 'e1lipsea form of change which is easily observableso that the resonance condition can be observed with great closeness.

In the modification shown diagrammatically in Fig. 2 one end of the tuned circuit I, 2 is connected to one input terminal 19 and to the cathode I3 of a thermionic valve 14 while the other end thereof is connected to the anode l5 of the valve, the grid 18 being connected to the remaining input terminal ll. One pair I, 8 of deflecting plates is connected across the input terminals I0, II and the other across the tuned circuit I, 2. With this arrangement the pattern on the screen will, at resonance, be a straight line, but for even very small departures from resonance, the straight line will become a narrow ellipse.

It has been found possible, in experimental practice with indicator instruments in accordance with this invention and using quite normal oscillatory circuits having a Q value of the order of 200 or less to obtain an indication of frequency to within 1 part in 10,000. r

The tuned circuit I, 2 in either of the embodiments illustrated will normally be tuned to resonate at the frequency applied at l0, H but this is not a necessary condition since frequency indication can still be obtained if a harmonic relationship exists between the input frequency and the frequency of the circuit I, 2. When such harmonic relationship so exists a Lissajou figure without loops will appear on the tube screen, the loops disappearing exactly as the harmonic condition is reached but appearing for slight mistuning on either side of the condition. Different harmonics give difierent Lissajou figures but the results obtained are typified by Figs. 3 and 4 in which Fig, 3 shows a figure (without loops) obtained on the tube screen exactly at a harmonic relation while Fig. 4 shows at a and b loops which appear at slight mistunings of I, 2 away from the exact harmonic condition.

What we claim is:

l. A wave meter comprising a parallel tuned circuit, an oscillograph capable of deflection in two angularly related directions, a connection including an impedance between one terminal of said tuned circuit and one input terminal of the wave meter, a connection between the other terminal of said tuned circuit and the other terminal of the wave meter, means for applying voltage set up across the tuned circuit to the -means for producing deflection in one direction in said oscillograph and means for applying the voltage at the input terminals to the means for producing deflection in the other direction in said oscillograph.

2. A wave meter comprising a parallel tuned circuit, an oscillograph capable of deflection in two angularly related directions, a valve having its anode-cathode space connected across said tuned circuit and its grid-cathode space connected across the input terminals of the wave meter, means for applying voltage set up across the tuned circuit to the means for producing deflection in one direction in said oscillograph, and means for applying the voltage at the input terminals to the means for producing deflection in the other direction in said oscillograph. V

3. Apparatus in accordance with claim 1, wherein said impedance included in the connection between one terminal of said tuned circuit and one input terminal of said wave meter comprises a resistance.

GEORGE MAURICE WRIGHT. CYRIL AUSTIN. 

